An Ode to Quantifying Infection Risk in Addition to Prevalence

When you’re studying parasites (or symbionts or pathogens), the prevalence of the parasite in the host population is one of the easiest response variables to measure. That’s not to say that it is easy; there are certainly a variety of methodological difficulties that crop up, and it can be expensive to run lots of blood tests if you’re looking at seroprevalence. But getting a prevalence estimate is certainly a lot easier than pinpointing when each host becomes infected (e.g., via mark-recapture methods) and/or calculating the actual risk of infection (i.e., the rate that susceptible hosts become infected = force of infection). For that reason, we often use prevalence as a response variable, and hope that we can infer things about parasite transmission based on those data. Sometimes, it works out great! For instance, in 1854, John Snow (the physician, not the Brother on the Wall) mapped the locations of Cholera cases in London. By pinpointing an area of high incidence on the map, he found a water pump that was probably an important source of infection in the epidemic. But do areas of high disease incidence or prevalence always occur in areas of high disease exposure?

Littorina littorea, the common periwinkle, is an abundant and widespread marine snail that hangs out in the intertidal zone (various levels of exposure to the air with the tides) and the subtidal zone (almost never exposed to air). Periwinkles are hosts for a few different trematode species, but for today, we’ll just focus on Cryptocotyle lingua, which infects snails, then fish, then shorebirds. Snails get infected when they consume trematode eggs from shorebird feces. ‘Loitering’ shorebirds are 6-20 times more likely to hang out in the high intertidal zone than the low intertidal zone, and as a result, the density of shorebird feces in the high intertidal zone is 70 times higher than in the low intertidal zone (Byers et al. 2015). Therefore, it is not surprising that when uninfected ‘sentinel’ snails were placed in field cages in the high and low intertidal zones, snails were four times more likely to become infected in the high intertidal zone (Byers et al. 2015). In fact, the probability that an uninfected snail would become infected in the low intertidal zone was effectively zero. That makes sense, because bird guano was almost never found in that zone.

So, when Byers et al. (2015) went out and sampled periwinkles in the high and low intertidal zones, they found way higher prevalences of infection in the high intertidal zone, where infection risk was high, right? WRONG! The prevalence of infection was much higher in the low intertidal zone, even though snails do not become infected there! How could that be?

First, let’s back up and talk about an important selection pressure in the low intertidal zone: predation. There are extreme size-dependent predation pressures in that zone that pretty much prevent small/young snails from living there. So, the only snails in the low intertidal zone are bigger/older snails. Big/old snails are much more likely to be infected by trematodes than small/young snails, because they have had longer to be exposed and become infected. But we know that the big snails aren’t becoming infected in the low intertidal zone, so where are they coming from? It may be that young snails hang out in the high intertidal zone, escaping predation but experiencing high infection risk, until they are big enough to safely live in the low intertidal zone. Once big enough, the snails migrate to that low zone, which provides better foraging opportunities, and the high density of big, infected snails results in high prevalences of infection (76% infection!) in an area that has effectively zero risk of infection. Isn’t that neat?!

So, as Byers et al. (2015) point out, “disease risk and prevalence patterns need not be tightly coupled in space.” I think that’s important to remember when we’re deciding what response variables we want to consider in ecological and epidemiological studies.



Byers, J.E., A.J. Malek, L.E. Quevillon, I. Altman, and C.L. Keogh. Opposing selective pressures decouple pattern and process of parasitic infection over small spatial scale. Oikos.

Unofficial ESA 2015 Parasite Ecology Cartoon Contest

ESA 2015 is just one month away! Last year, I had tons of fun judging a parasite ecology cartoon contest that no one knew they were participating in. I posted the results here. This year, I’m announcing the (technically second annual) Unofficial ESA Parasite Ecology Cartoon Contest.

Here’s how it works: myself and a top secret team of judges will be watching your symbiont-related talks and taking notes on your use of cartoons. Our favorite cartoonist will be awarded an almost entirely worthless prize (i.e., snail mail from yours truly, some publicity for your cool science, and bragging rights for a year). The cartoons don’t need to be funny! We’re just looking for cartoons that help communicate your work to the audience. That being said, anything punny is worth mega bonus points.

My minions and I should be able to make it to the majority of the parasite-related talks, but it’s logistically impossible for us to see them all. If you know you’re going to have some rocking cartoons and you want in on this highly prestigious contest, let me know in the comments or via email and I’ll make a special effort to come to your talk. This is particularly important if you’re in a session that isn’t parasite-themed.

To anticipate some questions:

Will the judges be participating in the contest? No!

Can I use cartoons from this site, if I use proper attribution? Yes!

Can the judges be swayed by offers of free beer or tenure-track faculty positions? No! (Except yes. So much yes.)

Good luck!!

ASP Teaching Parasitology Symposium

Sadly, I’m not going to the American Society of Parasitologists conference this year. I’m bummed, because I’m missing the awesome talks in the Teaching Parasitology Symposium and the Science Outreach in the Classroom and Beyond session. The talks include:




And because nothing is quite as beautiful as the marriage of math and parasites, my personal favorite:


If you’re going to ASP 2015 and you’d like to do a guest post about the things you learn from these cool talks, let me know! I’m sure I’m not the only person who would like to live vicariously through you.

S Car Go

Hi, Folks! I’m still traveling for the holidays, so I can’t do a full post this week. But here’s a punny cartoon! (This doesn’t have anything to do with symbionts, unless you count the fact that both snails and caterpillars have many symbionts.)

Also, remember to vote on the best Parasite Ecology cartoon of 2014! So far, no one has voted Gary for Snail President.

SCarGoGet it? 😛

Best Parasite Ecology Cartoon of 2014?

Happy New Year!!

Last year, I let you guys vote on my best parasite ecology cartoon of 2013. The winner was “Social Networking in Lemurs,” a cartoon about this study that painted lice on lemurs to infer lemur contacts.

So, which of my 2014 parasite ecology cartoons was the best? I’m opening up the voting for these candidates:

1. Endless blood meals for $6.95!

wildebeest buffet2. Feel the water on my exoskeleton


3. He’s on ‘todes


4. Oldest trick in the book

Fecundity Compensation

5. Vote Gary for Snail President


6. Pirate worms!

HMS Crayfish 2

7. Godzilla’s parasitoids


8. Niche markets


9. Elepunts


10. I want grand-seedlings!


Pick your top three favorites!